Curable silicone compositions are used as sealing agents, protective agents, coating agents, or the like for optical semiconductor elements in optical semiconductor devices such as light emitting diodes (LEDs). Examples of such a curable silicone composition include curable silicone compositions comprising: a straight-chain organopolysiloxane having at least two alkenyl groups and at least one aryl group in a molecule; a branched-chain organopolysiloxane represented by the average unit formula: (RSiO3/2)a(R2SiO2/2)b(R3SiO1/2)c(SiO4/2)d(XO1/2)e (in the formula, R are the same or different monovalent hydrocarbon groups, however, in a molecule, from 0.1 to 40 mol % of all R are alkenyl groups and 10 mol % or greater of all R are aryl groups; X is a hydrogen atom or an alkyl group; and a is a positive number, b is 0 or a positive number, c is 0 or a positive number, d is 0 or a positive number, e is 0 or a positive number, and b/a is a number from 0 to 10, c/a is a number from 0 to 0.5, d/(a+b+c+d) is a number from 0 to 0.3, and e/(a+b+c+d) is a number from 0 to 0.4); an organopolysiloxane having at least two silicon-bonded hydrogen atoms in a molecule; and a hydrosilylation reaction catalyst (see Patent Documents 1 and 2); and a curable silicone composition comprising: a straight-chain organopolysiloxane having at least two alkenyl groups and at least one aryl group in a molecule; a branched-chain organopolysiloxane represented by the average unit formula: (RSiO3/2)f(R2SiO2/2)g(R3SiO1/2)h (in the formula, R are the same or different monovalent hydrocarbon groups, however, in a molecule, 0.5 mol % or greater of all R are alkenyl groups and 25 mol % or greater of all R are aryl groups; and f, g, and h are numbers satisfying:
0.30≤f≤0.60, 0.30≤g≤0.55, f+g+h=1.00, and 0.10≤h/(f+g)≤0.30); an organopolysiloxane having on average at least two aryl groups and on average at least two silicon-bonded hydrogen atoms in a molecule; and a hydrosilylation reaction catalyst (see Patent Document 3).
In Patent Documents 1 and 2, as the branched-chain organopolysiloxane, a branched-chain organopolysiloxane having a siloxane unit represented by formula: R′2SiO2/2 (in the formula, R′ are the same or different, and alkyl groups having from 1 to 12 carbons or alkenyl groups having from 2 to 12 carbons) has not been specifically described. Furthermore, Patent Document 3 describes that the content of the branched-chain organopolysiloxane exceeding 150 parts by mass, per 100 parts by mass of the straight-chain organopolysiloxane, leads to reduction in adhesion of the resulting cured product to substrate and thus is not preferable.
In general, such a curable silicone composition is used by blending a phosphor to convert the emission wavelength from the LED; however, because of poor dispersibility of the phosphor, the phosphor aggregates, thereby causing a problem in that the light from the LED becomes uneven. Furthermore, another problem exists in that, when light is emitted continuously by applying a high current to an LED sealed with a cured product of such a curable silicone composition, the cured product is cracked.